1. Field of the Invention
This invention relates to high security locks, as used for instance to close protected enclosures such as bank safes and strongrooms.
2. Description of the Prior Art
To improve the security of protected enclosures, of their contents, and of their operating personnel, it is known to provide locking systems involving a code or a high security key with so-called time-lock devices which will only allow these locking systems to be operated during pre-programmed periods of time, even if the operator possesses the code or the key.
For instance, European Patent Application No. 86810154.8 filed on 8 Mar. 1986 discloses a conditional-opening lock governed by a weekly program that is preferably stored in the memory of a microprocessor. This program provides time-slots during which restrictions imposed on the opening of a door are lifted. To that end, the locking mechanism may be blocked or released by means of an electromagnetically controlled latching unit governed by the microprocessor. By blocking the operation of the locking mechanism, the latching unit imposes an opening ban which can be lifted only if the microprocessor finds that the actual time coincides with a time slot of the preset program.
The electronic and electromagnetic aspects of the locking system are designed to reduce the likelihood of a breakdown or failure to a minimum, since a breakdown or failure may have most unfortunate consequences. For obvious reasons, the lock unit is mounted on the inside of the enclosure's door. This means, however, that should a breakdown occur while the latching unit is blocking the locking mechanism it will no longer be possible to open the door. To get to the contents of the enclosure it then becomes necessary to breach the enclosure by "violent" means which may partly destroy it. This is an extremely costly and hazardous operation and possibly too risky since the contents of the enclosure are, at least for a while, left unprotected.